When fossil fuels such as coal and petroleum are used as an energy source, it has been believed that carbon dioxide which is by-produced causes global warming. In addition, when atomic energy is used, pollution may be caused by radioactive rays in some cases. In recent years in which environmental issues have been actively discussed, excessive dependence on the energies described above may result in serious problems.
On the other hand, since solar cells, which are photovoltaic devices in which sunlight is converted into electric energy, use sunlight as an energy source, the influence of solar cells on global environment is significantly small, and hence it has been expected that solar cells become more widely used.
Among various materials which are used for forming solar cells, a large number of solar cells formed using silicon are commercially available, and the silicon solar cells can be categorized roughly into crystal silicon solar cells made of single crystal silicon or polycrystal silicon and non-crystal (amorphous) silicon solar cells. Heretofore, for the solar cells, single crystal or polycrystal silicon has been widely used. Although the crystal silicon solar cells described above have a high conversion efficiency, which represents the performance of converting light (sunlight) energy into electric energy, as compared to that of amorphous silicon, since a large amount of energy and a long period of time are required for the crystal growth, the productivity is low, and hence the crystal silicon has been disadvantageous in view of cost.
On the other hand, amorphous silicon solar cells have a low conversion efficiency as compared to that of crystal silicon solar cells; however, since light absorption is higher than that of crystal silicon solar cells, various advantages are obtained. That is, for example, various types of substrates may be freely selected, and a larger cell area can be easily achieved. In addition, the productivity of amorphous silicon solar cells is high as compared to that of crystal silicon solar cells. However, since vacuum processes must be performed for the production, production facility-related cost has been still a serious problem.
Accordingly, in order to further reduce the cost, various solar cells made of organic materials instead of silicon-bases materials have been investigated. However, when organic materials as described above are used for this purpose, the conversion efficiency is very low, such as 1% or less, and in addition, the durability is also another problem. Among various investigations, an inexpensive solar cell using a porous semiconductor particles sensitized by a dye has been disclosed in Nature (353, pp. 737-740, 1991). This solar cell is a wet type solar cell, that is, an electrochemical photocell, in which a porous thin film made of titanium oxide is used as a photoelectrode, the porous thin film being spectral-sensitized using a ruthenium complex as a sensitizing dye. The advantages of this solar cell are that an inexpensive oxide semiconductor such as titanium oxide can be used, light absorption of the sensitizing dye can be performed in a wide visible wavelength region of up to 800 nm, the incident photon-to-current efficiency is high, and high energy conversion efficiency can be realized. In addition, since vacuum processes are not required, a large production facility and the like are not necessary.
However, since the electrochemical photocell as described above is a wet type cell, there have been problems such as degradation in properties due to leakage and evaporation of an electrolyte solution, and hence the reliability of this type of photocell may not be always satisfactory. In order to overcome the problems described above, a gel electrolyte composed of a polymer such as polyethylene oxide (PEO) impregnated with an electrolyte solution has also been proposed. However, since the viscosity of the electrolyte is high, and nanoscale oxide semiconductor particles are used for electrodes, the electrode pores are not easily filled with the electrolyte, and as a result, a problem of decrease in solar-energy conversion efficiency may arise due to insufficient conducive properties. In addition, since cross-linking of the above gel electrolyte is primarily performed by the use of relatively weak secondary interaction such as intermolecular forces between polymer chains so as to form the gel, when heat is applied thereto, a problem may also arise in that the gel is easily changed into a liquid form. Furthermore, in film formation, since a coating step and a step of removing a solvent having a low boiling point and a low viscosity must be performed, the productivity is disadvantageously degraded.
Hence, investigation of a chemical cross-linking type gel electrolyte has drawn a lot of attention in which a polyfunctional monomer is dissolved in an electrolyte solution and is then polymerized by applying exterior energy such as heat or active rays. The feature of this gel electrolyte is that since the viscosity of a solution containing the monomer before polymerization and a plasticizer is low, the electrode pores are easily filled with the electrolyte solution. In addition, when a monomer solution formed by dissolving the polyfunctional monomer in the electrolyte solution is injected into a device assembled beforehand and is then in situ gelled, a photovoltaic device can be obtained which has superior chemical bonding at an electrode interface and superior conductive properties. Furthermore, by using an ionic liquid, that is, a molten salt, instead of an electrolyte solution using a solvent, a gel electrolyte having no vapor pressure can also be realized.
However, since iodine functions as a polymerization inhibitor in a general radical polymerization method, when an electrolyte solution contains iodine redox as an electron transport carrier, a problem may arise in that gelation cannot be in situ performed.
In addition, even when the iodine redox is not contained as an electron transport carrier, degradation of a sensitizing dye and an electrolyte layer inevitably occurs by application of heat and active rays in polymerization, and hence the photoelectric conversion properties are disadvantageously degraded.
Hence, in consideration of the conventional problems described above, the present invention was made, and an object of the present invention is to provide a solid electrolyte having superior conductive properties and reliability, a photovoltaic device using the solid electrolyte described above, and manufacturing methods thereof.